Vehicles with internal combustion engines use catalytic converters to convert exhaust constituents and reduce regulated emissions. However, in some converters, conversion efficiency can be low at ambient temperatures. As such, various approaches have been used to increase exhaust heat during engine starting to thereby obtain earlier increased conversion efficiency.
One such approach is described in U.S. Pat. No. 6,725,830, which uses different ignition timing in different cylinder groups to increase total exhaust heat to the catalyst/exhaust, while still providing accurate idle speed control, among other features. In one example, the ignition timing of one group is significantly retarded after top dead center so that little torque is produces, but large quantities of heat are produced. Then, the remaining cylinders operate at higher load, which also increases exhaust heat, while retaining accurate torque control.
However, the inventors herein have recognized that under some conditions, depending on the engine firing order exhaust mixing and heat generation may be somewhat degraded. For example, depending on engine configuration, the firing order may pair some cylinders with the same ignition timing and others with disparate timings. While such operation may be used to reduce vibration under some conditions, there are other conditions where such operation may result in less even exhaust heating and less quantity of exhaust heat transferred to the emission control system devices.
As such, in one aspect, a method for operating an engine having at least a first and second cylinder, comprising: operating the first cylinder with a spark timing more retarded than a spark timing of a second cylinder; and operating one of said first cylinder and second cylinders with a first number of strokes per cycle different from other cylinders of the engine for at least one combustion cycle.
In this way, it is possible to change a combustion firing order to enable improved exhaust gas heating with cylinders having disparate ignition timing, at least under some conditions. In one example, at least one cylinder can be operated with a different number of strokes to vary the combustion firing order until a desired firing order is reached. In another example, multiple cylinders can be operated with varying numbers of strokes to achieve a new firing order. In still other example, cylinders can have the number of strokes of their combustion cycle sequentially varied until a desired firing order is reached.
Another advantage of the above operation is that in some cases, a cylinder can operate with a decreased number of strokes per combustion cycle, thereby further increasing heat flux to the exhaust. For example, cylinders with more ignition timing retard can be operated with a two-stroke combustion cycle, thereby about doubling the amount of heat flux to the exhaust from those cylinders and thus obtain faster catalyst temperature increases.